This application makes reference to and claims all benefits accruing under 35 U.S.C. Section 119 from an application entitled, xe2x80x9cApparatus and Method for Fabricating Multi-Period Optical Fiber Gratingxe2x80x9d, filed with the Korean Industrial Property Office on Dec. 28, 1999 and there duly assigned Ser. No. 99-64119.
1. Field of the Invention
The present invention relates generally to a long-period optical fiber grating used as a gain equalized filter, and more particularly, to an apparatus and method for fabricating gratings of different periods.
2. Description of the Related Art
Fiber gratings are periodic variations in a refractive index along the length of a fiber. It is possible to make fibers in which the refractive index varies regularly along their length. These fibers are called fiber gratings as they interact with light, and their effects on light passing through them depend very strongly on the wavelength.
In general, an optical fiber grating is a row of fine parallel lines, usually on a reflective surface. Light waves bounce off the lines at an angle that depends on their wavelength. Thus, the fiber grating is used as a filter for selecting a predetermined wavelength directed to the particular core of an optical fiber as well as eliminating or reflecting light at a particular wavelength by periodically inducing the variance of a refractive index of the optical fiber through the provision of ultraviolet light. Ultraviolet light creates fiber gratings by breaking atomic bonds in the fiber member. Typically, the optical fiber grating is classified into a short-period optical fiber grating and a long-period optical fiber grating depending on its period of time.
A short-period optical fiber grating reflects a specific wavelength for performing a filtering function. In contrast, long-period grating (LPG) devices selectively remove light at specific wavelengths by coupling light from one optical mode of a fiber to another mode propagating in the same direction, with very low back-reflection. In particular, the long-period grating is utilized to couple light from a core mode to a cladding mode in the range from tens of kilometers to hundreds of kilometers, during which light at specific wavelengths can be removed. In essence, the long-period grating devices serve as a gain equalized filter in an erbium-doped optical fiber amplifier.
When the long-period grating is utilized as a gain equalized filter, there are some instances where the long-period grating requires multi-period gratings. FIG. 1 is a simplified diagram illustrating an optical fiber F with different long-period grating patterns according to the conventional art. Referring to FIG. 1, the long-period grating patterns, g1, g2 and g3, exhibiting gratings of differing periods are disposed along the optical fiber F in the direction of one axis. In this case, the notation, F1, F2 and F3 along the optical fiber represent the respective long-period grating patterns that are interconnected for use as a gain equalized filter.
The prior art system has some drawbacks as the gain equalizing long-period grating patterns have to be interconnected using a fusion splicer (or other similar alternatives). Moreover, the gain equalizing long-period grating patterns require an additional means, such as a contraction tube to secure the connection as the connected regions are easy to break. Furthermore, concise accuracy and costly high-tech equipment are required for the above interconnection process.
It is, therefore, an object of the present invention to provide an apparatus and method for fabricating a multi-period grating device using a multi-period amplitude mask with a plurality of on-off ratios.
It is another object of the present invention to provide an apparatus and method for fabricating a multi-period grating device with gratings of differing periods formed on a single optical fiber using a multi-period amplitude mask.
It is still another object of the present invention to provide an apparatus and method for fabricating a multi-period grating device in an economical way.
To achieve the above objects, there is provided an apparatus for fabricating a multi-period optical fiber grating, which includes: an optical source; an optical fiber having a first part of the optical fiber and at least one second part of the optical fiber continuously connected to the first part and arranged substantially in a parallel relationship with the first part of the optical fiber; and a multi-period amplitude mask disposed between the optical source and the optical fiber, wherein the multi-period mask comprises the first sector with a first on-off ratio corresponding to the first part of the optical fiber and at least one second sector being substantially parallel with the first sector with a second on-off ratio, and wherein gratings of differing periods can be fabricated by passing a light beam through the periodic first and second sectors of the mask.